1. Field of the Invention
The invention relates to a bulk nitride mono-crystal especially for use as a substrate for epitaxy. Such a substrate for epitaxy is particularly suitable for preparation of nitride semiconductor layers in a process for manufacturing of various opto-electronic devices.
Known nitride-based opto-electronic devices are manufactured on sapphire or silicon-carbide substrates, differing from the thereafter deposited nitride layers (i.e. heteroepitaxy).
2. Description of the Related Art
In the most commonly used Metallo-Organic Chemical Vapor Deposition (MOCVD) method, GaN depositing is performed from ammonia and metallo-organic compounds from the gaseous phase, and the growth rates attained make it impossible to receive a bulk layer. However, MOCVD cannot produce a bulk crystal having a substantial thickness. In order to reduce surface dislocation density a buffer layer is first deposited on sapphire or silicon substrate. However, the reduction of surface dislocation density achieved is not bigger than to about 108/cm2.
Another method that has been proposed for the manufacturing of bulk mono-crystalline gallium nitride, involves epitaxial depositing using halogens in the gaseous phase and is called Halide Vapor Phase Epitaxy (HVPE) [“Optical patterning of GaN films” M. K. Kelly, O. Ambacher, Appl. Phys. Lett. 69 (12) (1996) and “Fabrication of thin-film InGaN light-emitting diode membranes” W. S. Wrong, T. Sands, Appl. Phys. Lett. 75 (10) (1999)]. The method allows generation of GaN substrates 2 inches in diameter, but the quality is insufficient for application in laser diodes, because the surface density of defects still remains in the 107 to 109/cm2 range. Besides, the HVPE GaN substrates have tilted crystal axes because of distortion caused by epitaxial growth on hetero-substrates, for example on sapphire.
Recently, defect density decrease is attained by using the Epitaxial Lateral Overgrowth (ELOG) method. In this method, a GaN layer is first grown on the sapphire substrate, and then SiO2 is deposited in the form of strips or grids. Next, such a substrate may be used for lateral GaN growing, reducing the defects density to about 107/cm2.
Due to significant differences in chemical, physical, crystallographic and electrical properties of substrates such as sapphire or silicon carbide and semiconductor nitride layers deposited thereon by hetero-epitaxy, big technological effort is needed to advance progress in opto-electronics.
On the other hand growth of bulk crystals of gallium nitride and other nitrides of Group XIII elements is also extremely difficult (numbering of the Groups is given according to the IUPAC convention of 1989 throughout this application). Standard methods of crystallization from alloy and sublimation methods are not applicable because of decomposition of the nitrides into metals and N2. In the High Nitrogen Pressure (HNP) method [“Prospects for high-pressure crystal growth of III-V nitrides” S. Porowski et al., Inst. Phys. Conf. Series, 137, 369 (1998)] decomposition is inhibited by applying a nitrogen atmosphere under high pressure. Growth of crystals is carried out in melted gallium, i.e. in the liquid phase, resulting in production of GaN platelets about 10 mm in size. Sufficient solubility of nitrogen in gallium requires temperatures of about 1500° C. and nitrogen pressures of the order of 1500 MPa.
In another known method, supercritical ammonia was proposed to lower the temperature and decrease pressure during the growth process. It was proven in particular that it is possible to obtain crystalline gallium nitride by synthesis from gallium and ammonia, provided that the latter contains alkali-metal amides (KNH2 or LiNH2). The processes were conducted at temperatures of up to 550° C. and pressure 500 MPa, yielding crystals of about 5 μm in size [“AMMONO method of BN, AlN, and GaN synthesis and crystal growth” R. Dwiliński et al., Proc. EGW-3, Warsaw, Jun. 22–24, 1998, MRS Internet Journal of Nitride Semiconductor Research, http://nsr.mij.mrs.org/3/25].
Use of supercritical ammonia also allowed recrystallization of gallium nitride within the feedstock comprising finely crystalline GaN [“Crystal Growth of gallium nitride in supercritical ammonia” J. W. Kolis et al., J. Cryst. Growth 222, 431–434 (2001)]. Recrystallization was made possible by introduction of amide (KNH2) into supercritical ammonia, along with a small quantity of a halogen (KI). Processes conducted at 400° C. and 340 MPa gave GaN crystals about 0.5 mm in size. However, no chemical transport processes were observed in the supercritical solution, in particular no growth on seeds.
The thus obtained nitride mono-crystals are of no industrial use as substrates for epitaxy, mainly because of their insufficient size and irregular shape.
Lifetime of optical semiconducting devices depends primarily on crystalline quality of the optically active layers, and especially on surface dislocation density. In case of GaN-based laser diodes, it is beneficial to lower dislocation density in the GaN substrate layer to less than 106/cm2, and this is extremely difficult in the methods used so far. On the other hand industrial processes for manufacturing such optical semiconducting devices can be performed only on reproducible substrates meeting strict quality specifications.